(1) Field of the Invention
The present invention relates to a conductive synthetic resin molding material which, upon being molded, causes only a slight degradation in the mechanical strength of the synthetic resin, and which has an excellent electromagnetic shielding effect.
(2) Description of the Prior Art
To protect electronic circuits from external disturbance and to prevent the undesired leakage of electromagnetic waves from an oscillation circuit or the like, the casing of such electronic equipment must be made of an electromagnetic shielding material. The electromagnetic shielding material may be a metal, a conductive synthetic resin or the like. Although the former has an excellent electromagnetic shielding effect, it is disadvantageous, in that it is heavy, expensive and has poor workability. For this reason, the latter, a conductive synthetic resin, is currently in widest use.
One method of rendering a synthetic resin electrically conductive includes the technique of forming a conductive layer on the surface of a synthetic molded product, either by application of a conductive paint or by spraying or plating a metal onto the synthetic resin product, as well as the internal addition technique of adding a conductive filler such as carbon, a metal powder or a fiber to the synthetic resin material itself. The former method requires a large number of steps and does not allow for easy mass-production. Furthermore, the conductive layer formed by this method may eventually peel after a long period of time. For this reason, the latter method is expected to be more practical. Despite its advantages, however, the latter method is also subject to the following problems. First, to obtain a desired electromagnetic shielding effect, a large amount of a conductive filler, such as carbon or a metal, must be mixed with the resin. This frequently results in poor dispersion of the filler or a low mechanical strength of the final product. Furthermore, when a metal or the like is added to the resin, the synthetic resin is degraded in its various properties. In addition to this, a metal fiber or a fibrous filler with good flexibility tends to become entangled in its initial form in a single substance. Thus, such a filler must be disentangled before addition to the resin. Uniform milling of the resin, together with such a filler, requires advanced skill and technology, due to differences in specific gravity and shape. An operator may also suffer from pain or itchiness in handling the fiber, which presents a problem in the working environment.